The invention relates to a vehicle with at least one cooling circuit for cooling a fuel cell system.
A fuel cell system for a vehicle is disclosed in German Patent Document DE 196 29 084 C2. The fuel cell system includes a primary cooling heat exchanger, which, analogously to vehicles with internal combustion engines, is arranged in such a way that the dynamic pressure of the motion-related airflow ensures throughflow of ambient air as cooling air. In order to make use of the dynamic pressure, basically only the front surfaces of the vehicle can be used for the cooling heat exchangers. This involves corresponding limitations and disadvantages, which are described in the following paragraphs, together with other problems.
The heat dissipation during operation of fuel cell drive systems in the case of PEM fuel cell applications represents a problem that places a limitation on performance, as the greater part of the waste heat that is generated has to be dissipated into the environment via the cooling circuit where there are only small differences in temperature. In system designs that are suitable for use in practice, as a rough starting point it can be assumed that around the same level of waste heat has to be dissipated via the cooling circuit as electrical power is generated in the fuel cell.
Heat dissipation of the cooling circuit can in principle be improved by enlarging the cooling surface, improving the throughflow of the cooling heat exchanger or by raising the temperature of the cooling medium. Cooling concepts developed to date according to the general state of technology make use of one or several of these aspects, but are associated with the disadvantages or restrictions described below.
The maximum operating temperatures of the PEM fuel cell stacks of today lies at around 80-90° C. Efforts to increase the temperature affect the durability (lifetime) of the fuel cell and from today's point of view can be considered as a long-term target. In addition, water-based cooling circuits only permit very limited further increases in temperature, while cooling media capable of functioning at high temperature (e.g., thermo-oils) have higher sealing requirements, increased costs and more complex handling. The raising of the front surface of the cooling heat exchanger, and only in this situation does the effect described in the aforementioned DE 196 29 084 C2 with regard to the dynamic pressure occurs subject to severe limitations in automotive engineering. The main limiting factors are package and design specifications, and also the desire to achieve the lowest-possible air resistance factor (cw.A-Wert) in the interests of low vehicle energy consumption. Improvement of cooling effects using high-performance fans also brings serious disadvantages. The necessary drive power for the fans leads to a considerable increase in parasitic losses. This means that the efficiency rating of the system is reduced. In addition, vehicle acoustics suffer negative impact.
For these reasons, the heat dissipation capability of the cooling circuit presents a limiting factor for the electric performance of mobile fuel cell systems in vehicles. In order to provide a solution to these problems in the fuel cell systems in vehicles, German Patent Document DE 10 2005 021 413 A1 discloses the use of additional surfaces in the area of the vehicle as a cooling surface. However, the design of the vehicle is then changed accordingly, which is often not desired.
Furthermore, a cooling circuit for a fuel cell vehicle is described in U.S. Pat. No. 6,370,903 B1, which uses a high-temperature cooling circuit with a cooling heat exchanger utilizing the motion-related airflow for cooling of the fuel cell itself. In addition, for cooling of the drive motor and of the electrical and electronic components of the vehicle, a low-temperature cooling circuit is shown in a known form that is also cooled by a cooling heat exchanger utilizing the motion-related airflow.
Vehicles are currently equipped with a climate control device which, according to the embodiment described here, can be used for climate control of an interior of a vehicle and for support of the cooling circuit of the fuel cell. The installation is shown as a heat pump.
Basically it is the case that a climate control system also has to be provided with a climate control cooling heat exchanger in order to cool or condense the climate control medium used in the climate control system. The installation is of such a type that very often this climate control cooling heat exchanger is integrated into the cooling circuit of the fuel cell, or, as described in U.S. Pat. No. 6,370,903 B1, is integrated into the low-temperature cooling circuit for the electronic components.
In such integration, in the case of use of the climate control equipment for cooling the interior of the vehicle, the disadvantage always occurs that the amount of heat entering the cooling circuit is further increased so that, in particular in the presence of very high ambient temperatures, the cooling of the fuel cell stack is worsened further. Because of this worsening of the cooling of the fuel cell stack, the performance capability of the vehicle equipped with the fuel cell system is correspondingly reduced. The supporting cooling of the fuel cell cooling circuit mentioned in U.S. Pat. No. 6,370,903 B1 by means of an evaporator of the climate control device, exhibits the disadvantage that such cooling is associated with a comparatively high use of energy in the climate control device, so that the overall efficiency rating of a vehicle thus equipped suffers.
Exemplary embodiments of the present invention provide a vehicle with at least one cooling circuit for cooling of a fuel cell system that enables a maximum cooling performance with the minimum energy requirement, and therefore allows high performance capability of the fuel cell system, even in the case of difficult ambient conditions.
Because the cooling heat exchanger in the vehicle according to the invention is designed with at least two stages, the surface available for cooling in the cooling heat exchanger is considerably increased. The arrangement of the at least two stages of the cooling heat exchanger so that they are blown through by the motion-related airflow as cooling air serially, one after another, means that the at least two stages can be used without the necessary flow surface having to be greater. The installation can therefore be integrated into existing vehicle concepts without problem, as only the thickness of the entire cooling heat exchanger or the stack of the stages of the cooling heat exchanger in the direction of driving is increased because of the higher number of stages, without the need for a greater flow surface.
According to one embodiment of the vehicle according to the invention, the at least two stages of the cooling heat exchanger are flowed through by a cooling medium flowing in the cooling circuit serially, one after another, whereby the stage last flowed through by the motion-related airflow is flowed through first by the cooling medium. As the individual stages of the cooling heat exchanger are arranged one behind the other in the direction of travel, these are also flowed through after one another by the motion-related airflow as cooling air. This means that in the individual stages there is a different temperature differential between the cooling medium to be cooled in the respective stage and the motion-related airflow as cooling air which cools this cooling medium. Correspondingly, the individual stages of the cooling heat exchanger can be cooled to different levels. Because the individual stages are arranged in such a way that they are flowed through serially by the cooling medium, it can now be achieved that the stage of the cooling heat exchanger that is flowed though by the warmest cooling medium is already cooled by the warmest motion-related airflow, in other words the motion-related airflow that has already been heated by the other stage. This means that the cooling of the cooling medium is implemented in the best possible way, as the cooling medium that is still very warm is cooled by comparatively warm motion-related airflow, so that there is a sufficient difference in temperature in order to achieve at least preliminary cooling of the cooling medium in the first stage (from the point of view of the cooling medium) and in the last stage (from the point of view of the motion-related airflow). In the one or the subsequent stages, the cooling medium then will be progressively colder, in the same way as the motion-related airflow, so that a complete cooling of the cooling medium to the temperature level which is needed for the full performance capability of the fuel cell system can be attained.
In an advantageous further development of the vehicle according to the invention, the cooling circuit includes a first section as high-temperature cooling circuit for cooling of the fuel cell stack, whereby in parallel to the cooling heat exchanger a second section is designed as a low-temperature circuit for cooling of electrical and/or electronic components, whereby a low-temperature cooling heat exchanger is provided and located in the cooling circuit parallel to the at least two-stage cooling heat exchanger in such a way that the low-temperature cooling heat exchanger is flowed through by the motion-related airflow serially to the at least two stages of the cooling heat exchanger. According to this particularly advantageous further development of the vehicle according to the invention, a low-temperature cooling heat exchanger in the form of a further stage from the point of view of the motion-related airflow is arranged serially to the at least two stages of the cooling heat exchanger. Accordingly, the installation facilitates integration of the low-temperature cooling circuit that is already known and generally used according to the state of technology in fuel cell driven vehicles for cooling of electrical and/or electronic components such as for example, the drive motor and the power electronic components, into the actual cooling circuit. The low-temperature cooling circuit is then designed as a second section of the cooling circuit, so that additional tube elements and also possibly an additional cooling medium transportation device are not required.
According to one of the possible embodiments of the vehicle, the vehicle also includes a climate control device. In the embodiment according to the invention, it is now particularly advantageous if the climate control device includes at least one climate control heat exchanger, in order to cool the climate control medium used in the climate control device, whereby the climate control heat exchanger is designed so as to be independent of the cooling circuit for the fuel cell system. This independence of the climate control cooling heat exchanger for cooling or condensing of the climate control medium of the climate control device from the cooling circuit of the fuel cell system, ensures that no additional heat is taken into the cooling circuit for the fuel cell system via the cooling of the climate control medium. It is rather the case that this heat, which occurs particularly in the presence of high ambient temperatures when cooling of the fuel cell system is in any case difficult, is cooled down elsewhere, so that this additional heat does not exercise a negative influence on the fuel cell system or its cooling.
This means that the fuel cell system can be cooled independently of the climate control device and can retain its performance capability.
In a particularly favorable further development of this, the at least one climate control heat exchanger is arranged in or in front of a wheel arch of the vehicle. This arrangement in the wheel arches of the vehicles can be completely independent of the cooling surface of the at least two-stage cooling heat exchanger of the cooling circuit for the fuel cell system, without removing any of the surface available to it over which a dynamic pressure of the motion-related airflow flows. In addition, the arrangement in or in front of the wheel arches to a large extent excludes negative influence on the appearance of the vehicle, so that attention is not drawn to these additional climate control cooling heat exchangers or only minimal attention is drawn to them.
Further advantageous embodiments of the invention can be found in the remaining dependent claims and become clear from the example of the embodiment which is explained in more detail below with reference to the drawings.